How many terms of AP 17 15 13 11 must be added to get some 72 ?explain

1.	How many terms of AP 17 15 13 11 must be added to get some 72 ?explain that answer
Answer» We are given a = 17, d = -2, Sn\xa0= 72 and we have to find n.Using Sn\xa0= n[2a +(n -1)d]/2, we get72 = n[2.17 +(n -1)(-2)]/2 = n(36 -2n)/2 = n(18 -n)=> n² -18n +72 = 0 => (n -6)(n -12) = 0=> n = 6, 12Both values of n, being positive integers are valid.\xa0We get double answer because sum of 7th to 12th terms is zero, as some terms are positive and some are negative.

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